What is the ruling on the prayer of a person afflicted with urinary incontinence who uses a bag and a pad, and if he sleeps and an erection occurs and protrudes from his underwear, does he have to change his clothes or wash his body?

If the body or clothes become impure with what you mentioned, it is obligatory to purify the impure part of the body or clothes, or to change the impure clothes. It is recommended to designate special clothes for sleeping to avoid the hardship of frequent changing. However, if it is merely a doubt or a whisper (waswasa), the original state is the purity of the body and clothes, and one does not deviate from this original state merely by doubt.